This invention relates to a microwave coupling system and, more particularly, to a system by which a symmetrical mode can be excited or received and two orthogonal asymmetrical modes at two frequency bands can be excited or received.
Antenna feed systems capable of generating and receiving microwave power in a plurality of modes have been developed and are known as multimode feed systems. Such multimode feed systems are often used in monopulse tracking antennas wherein the energy transmitted and received by the feed systems is combined in such a manner that sum (symmetrical) and difference (asymmetrical) mode radiation patterns are produced during transmission and/or reception. These patterns are analyzed to determine the position of a passive (reflecting) or active (radiating) object which may be either an aircraft, a missile, or a satellite or celestial body or to provide automatic tracking of these objects. Monopulse tracking systems are discussed for instance in, "Radar Handbook," by Merrill I. Skolnick, published 1970 by McGraw-Hill Book Co. and "Introduction to Monopulse," by D. R. Rhodes, published in 1959 by McGraw-Hill Book Co.
The typical tracking feed system may include several horns or apertures. When only a small number of horns are used, such as in the four-horn antennas, the radiation patterns have undesirable characteristics mainly in the form of high level sidelobes and internal losses which lower the efficiency (tracking slope) and increase the noise temperature of the system. Some prior art single aperture monopulse couplers although operative and possessing improved tracking slope have lower than ideal gain to noise temperature ratio for their sum mode when they are used as feed systems for reflector-type antennas and when operated over a wide range of frequencies. For more details on a single aperture monopulse coupler, see pages 21-18 through 21- 25 in the previously-cited "Radar Handbook."
One type of multimode coupler by which sum and difference modes can be launched into the throat of a single aperture horn is described in applicant's U.S. Pat. No. 3,560,976. It is desirable in certain applications such as in frequency reuse systems that higher gain over noise temperature (loss) ratios and particularly lower cross-polarization levels for the associated sum mode operation be provided. In frequency spectrum reuse applications for communication systems, the same frequency spectrum is reused but is communicated at orthogonal polarizations. In such systems the total information carrying capacity of the system is improved by increasing the isolation between the two approximately orthogonal polarizations. The isolation, of level difference, between the two polarizations is usually maximum in the direction represented by the symmetry axis of the main beam. It is therefore highly desirable to achieve an accurate alignment of the antenna axis toward the other terminal of the link (antenna of a satellite for example) by a high quality orthogonal difference mode to permit tracking. This however has to be done with minimum noise temperature (loss) contribution from the tracking circuit to the communication circuit and by minimum depolarization effect from the tracking circuit itself of the sum channel circuit. Furthermore in spectrum reuse systems, the tracking capability is desirable at one of two orthogonally polarized and different beacon frequencies.
The above problems have been partially overcome by a multimode coupler system including a funnel shaped coupler as described in applicant's U.S. Pat. No. 3,936,838, dated Feb. 3, 1976. Briefly, the system includes a funnel-shaped hollow member with a small aperture end of the funnel-shaped member adapted to pass symmetrical mode signals and the large aperture end adapted to be coupled to free space or to the throat of a horn radiator. Asymmetrical mode coupling is provided to a plurality of side wall coupling apertures in a given plane with these side wall apertures located a given length from the small aperture end of the funnel-shaped member. This given length is made equal to approximately one-half the guide wavelength of the TE.sub.21 asymmetrical mode or multiple thereof at the desired coupling frequencies. A difficulty occurs if one wishes to operate the system using two or more beacon tracking frequencies where these frequencies are fairly close to each other such that it is difficult to physically separate these side wall coupling apertures or their associated circuitry. Also, it may be desirable to operate the coupler at one of the beacon frequencies in a left circuitry polarized or linearly polarized mode while operating at the other beacon frequency in a right circularly polarized or orthogonally polarized mode or to operate the system in a more broadband mode. The present invention is aimed at solving these problems.